One example of a halogen gas laser device of this type is proposed in Japanese Patent Application (OPI) No. 6384183 (the term "OPI" as used herein means an "unexamined published application").
An excimer laser device uses ArF, KrF, XeCl or XeF as its laser active medium. In the excimer laser device, pulse discharge is caused between the anode and the cathode in a mixture of inert gas such as Ar, Kr or Xe and halogen gas such as F.sub.2 :, NF.sub.3 :, Cl.sub.2 or HCl or halogen compound gas, to generate a laser beam. The service life of the excimer laser device is adversely affected by factors such as deterioration of the laser gas, deterioration of the laser reflection mirrors, deterioration of the switching element, deterioration of the main capacitor, and deterioration of the preliminary ionization, etc. Against each of these factors, efforts have been developing to lengthen the service life of the excimer laser device. Since the above-described inert gases are expensive in general and, therefore, the excimer laser device is so designed as to operate in a sealed-off tube.
However, since the halogen gases are high in chemical reactivity, a remarkable chemical reaction occurs at the electrodes thereby deteriorating the gas.
FIG. 1 is a rough view showing the above-described excimer laser device having a gas reproducing system. The halogen gas used in a laser oscillation section 1 is circulated, in a sealing mode, through a laser gas circulating path 2, a dust filter 3, a circulating pump 4, an ultraviolet-ray applying section 6, and a low-temperature trap 5.
FIG. 2 is a partial perspective view, with parts cut away, showing the ultraviolet-ray applying section 6 in the gas circulating path. As shown in FIG. 2, a gas pipe 6b and an ultraviolet-ray generating lamp 6c are arranged in parallel with each other in an elliptic-cylinder type reflecting mirror 6a, and the electric power from a power source 6d is supplied to the ultraviolet-ray generating lamp 6c.
The halogen gas used by the laser oscillation section 1 is a mixture of hydrogen chloride (HCl), xenon (Xe) and helium (He). The wavelength of the output laser beam is 308 nanometers. These gases react and change at the electrode section. A part of the hydrogen chloride gas is decomposed into hydrogen gas (H.sub.2) and chlorine gas (Cl.sub.2), as a result of which the hydrogen chloride gas density is lowered and, accordingly, the intensity of the laser output is lowered.
The deteriorated gas containing the hydrogen gas and chlorine gas which flows along the laser gas circulating path 2 is sent to the dust filter 3, where solid materials are removed therefrom. The gas thus treated is delivered to the ultraviolet-ray applying section 6 by the circulating pump 4. When being exposed to the ultraviolet-rays at the ultraviolet-ray applying section 6, the hydrogen (H.sub.2) and chlorine (Cl.sub.2) react as follows: ##STR1##
Thus, hydrogen chloride gas HCl is reproduced. The gas HCl thus reproduced is sent to the low-temperature trap 5, where high boiling point impurities are removed from it. The gas thus treated is returned to the laser oscillation section 1.
The proposed gas control device for a halogen gas laser is constructed as described above. Therefore, in the case of the excimer laser device using a hydrogen chloride gas (HCl) as its halogen gas, the hydrogen gas (H.sub.2) and chlorine gas (Cl.sub.2) formed by decomposition are used to reproduce a hydrogen chloride gas (HCl), whereby the service life of the operating gas can be somewhat lengthened. On the other hand, in the case of the excimer laser device using a fluorine gas (F.sub.2) still suffers from the following difficulty: In the excimer laser device, fluoride is formed through electric discharge between the electrodes, and therefore it is difficult to reproduce a fluorine gas in order to increase the service life of the laser gas.